In drilling bore holes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters are employed. The bit is secured to the lower end of a drill string that is rotated from the surface, or the bit is rotated by downhole motors or turbines. The cutters or cones mounted on the bit roll and slide upon the bottom of the bore hole as the bit is rotated, thereby engaging and disengaging the formation material to be removed. The rolling cutters are provided with cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight of the drill string. The cuttings from the bottom of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drill string.
The earliest rolling cutter, earth boring bits had teeth machined integrally from steel, earth disintegrating cutters. These bits, typically known as “steel tooth” or “milled tooth” bits, are used for penetrating the relatively soft geological formations of the earth. The strength and fracture toughness of steel teeth enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths. However the same cutting structure that drills sand formations fast, slows down considerably when it encounters shales. This is due in part to the shale sticking to the bit when it cannot be readily removed by the drilling fluid because of the chisel shape of the teeth and their location on the bit.
It has been common in the arts since at least the 1930s to provide a layer of wear-resistance metallurgical material called “hardfacing” over those portions of the steel teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast, or macrocrystalline tungsten carbide dispersed in a steel matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced and applying the hard particles to the matrix to form a uniform dispersion of hard particle in the matrix.
Typical milled tooth bits have their teeth milled such that the inner and outer ends and leading and trailing flanks are fairly wide flat surfaces. The flat wide surfaces normal to the direction of rotation increase the tendency for the bit to ball up when sliding in shales. Typical hardfacing deposits are welded over a steel tooth that have a shape similar to the shape of the underlying tooth.